Corrosion resistant austenitic steel

ABSTRACT

D R A W I N G A substantially non-porous, sulfuric acid resistant austenitic stainless steel and a method for making it are disclosed. The steel includes from 15-45% manganese, from 10-30% chromium, from 1-3% copper, from 0.85-3% nitrogen, from 0-2% silicon, from 0-1% carbon, and the balance iron and residuals. In addition to containing the foregoing elements within the above-noted composition ranges, the alloys must be such that:

[ Sept. 23, 1975 CORROSION RESISTANT AUSTENITIC STEEL [75] Inventor:Albert G. Hartline, III,

Brackenridge, Pa.

Allegheny Ludlum Industries, lnc., Pittsburgh, Pa.

22 Filed: Apr. 30, 1973 21 Appl. No.: 355,394

[73] Assignee:

[52] U.S.'Cl 75/122; 75/125; 75/126 .1 [51] Int. Cl C22c 39/14; C22c39/26 [58] Field of Search 75/125, 122, 126 .I

[56] References Cited UNITED STATES PATENTS 2,184,305 12/1939 Kropt75/125 2,198,598 4/1940 Becket..... 2,850,380 9/1958 Clarke 75/1252862812 12/1958 Dulis 75/125 3,112,195 11/1963 Souresny 75/125 FOREIGNPATENTS OR APPLICATIONS 778.597 7/1957 United Kingdom 75/126 R PrimaryExaminerL. Dewayne Rutledge Assistant Examiner-Arthur J. SteinerAttorney, Agent, or Firm-Vincent G. Gioia; Robert F. Dropkin [5 7]ABSTRACT A substantially non-porous, sulfuric acid resistant austeniticstainless steel and a method for making it are disclosed. The steelincludes from 15-45% manganese, from 10-30% chromium, from l3% copper,from 0.853% nitrogen, from 0-2% silicon, from 01% carbon, and thebalance iron and residuals. In addition to containing the foregoingelements within the above-noted composition ranges, the alloys must besuch that:

30 (%C %N) 0.5 (%Mn) 1 5 %Cr+ 1.5 (%Si) (2) %Cr+ 0.8 (%Mr1) 11.88 (%N0.1) (28.5 %Cu) 5 0 i 7"Claims, 7 Drawing Figures BACKGROUND or THE'INVENT'IQN,"

Corrosion resistant steels known as stainless steels have long beenknown and are presently available with a variety of properties.Austenitic stainless steels, which are those consisting substantially ofasingle austenite phase, possess the best properties of corrosionresistance and good mechanical properties, particularly at hightemperature.

Austenitic stainless steels in the past have been steels in whichchromiumand nickel are the principal alloying agents. However, nickel isnot an abundant metal and the increased demand for it has increaseditsprice and made its supply uncertain, particularly in critical times.Substitutes for nickel in the chromium-nickel austenitic stainlesssteels have long been sought. Recently the combined use of manganeseandnitrogen along with chromium in carefully limited ratios has producedan austenitic stainless steel. In that steel, at least 0.85% nitrogen isrequired to produce the austenitic structure but large amountsof'nitrogen cause the steel ingot to be porous. Accordingly, it has beenfound that the nitrogen content of such a steel must be limited to lessthan 3%. Throughout the specification and claims all references topercent in composition shall be percent by weight of the totalcomposition.

The manganese in the above-noted steel must be present inamounts of from15-45%. Manganese is necessary to increase the solubility of nitrogen insteel, in addition to itself contributing to the austenitic structure. Ii

Although the chromium-manganese-nitrogen stainless steel is an excellantaustenitic stanless steel, it has been found to be subject to attack bysulfuric acid to a significant extent and accordingly its use is limitedto those environments where sulfuric acid is not present.

SUMMARY OF THE INVENTION The present invention is achromium-manganesecopper-nitrogen alloy that is non-porous, austenitic,and highly resistant to attack by sulfuric acid. The alloy of thisinvention contains from 1'5-45 manganese, from 10-30% chromium, froml-3% copper, from 0.853% nitrogen, from -2% silicon, from 0l% carbon,and the balance iron and residuals Residuals are impurities in the ironthat are not deliberately added to the alloy and have no significantdetrimental effect in the alloy.

In addition to the forgoing ranges, the c'ompo' siton of the alloys mustbe balanced in accordance withthe following equations: 7

%Cu) 2 0 g I (2) In the alloys of the presentinvention, chromium must bepresent to produce the."same effect that it does in prior art alloys.The alloys of this invention must con-- tain from 10-30% chromium- Atleast 10% chromium is required to give the steel itsoutstandingcorrosion resistance. Chromium also has a secondary effectupon the strength Of the steel and is a primary element; in increasingthe steels solubility for nitrogen. When more than chromium is presentin the alloy, a ferrite phase is formed which degrades the mechanicalproperties of the steel and accordingly more tha 30% chromium should notbe present. A preferred chromium content is in the range from 15 -27% inthat steels containing this range of chromium are easy to processManganese in excess of produces no further beneficial effectand shouldbe avoided as wasteful and because large quantities of manganese in analloy tend to attack furnace refractories.

Nitrogen is a strong austenitizer and should be present in the steel inamounts from 0.85 -3%. At least 0.85% is requiredto provide theaustenitic structure of the steel while amounts of nitrogen in excess of3% tend to yield porous ingots which are not-satisfactory. The nitrogencontent of the alloy of this invention preferably is from 1.05 1.5%.

The copper content of the alloy of this invention must be between 1 3%.Although copper has been known as an alloy additive to increaseresistance to sulfuric acid attack, it has in the past been used innickelcontaining alloys and in large amounts. The copper of previouslyknown acid-resistant alloys has caused a sacrifice in other properties.In the present invention relatively'little copper is effective toproduce the sulfuric acid resistant alloy, and the alloys of thisinvention have substantially the same mechanical properties as theircounterparts containing no copper or only residual amounts. In fact,alloys of this invention containing up to 1.5% copper have been found tobe hot-workable so that they may be rolled,'forged or otherwise shapedand their'properties altered for specific uses that could not be readilyobtained in the as-cast condition.

Carbom of course, is a well-known austenitizer and strengthener forsteels and. is employed in the alloys of this invention in amounts up to1%. The concentration of carbon must be maintained below that level inthat larger amounts remove chromium from the solid solu-. tion byforming chromium carbide, which requires higher annealing temperaturesto dissolve the carbide. It is preferred that less than 0.15% carbon bepresent inthe alloy of this invention. 1 r

The alloys of the present invention may tolerate silicon concentrationsas high as 2% but preferably the silicon is below 1%. Higher quantitiesof silicon tend to remove manganese from the alloy in the form ofmariganese silicates and tend to form inclusions in the steel.

Although the residuals inthe iron need not be identified and do notsignificantly affect the properties of the alloy, the usual residualsmaybe identified as molybdenum, phosphorus, sulfur, tungsten, cobalt andnickel.

Since the stainless steel composition of this invention is desirably asubstantially one-phase austenitic material, thermal treatments thattend to precipitate other phases .should beavoided. Although the alloysof this invention are not particularly sensitive to precipitation ofother phases, the method ofpreparation employed should avoid long dwellperiods in the 1000-l600F temperaturerange. Long dwelll periods would becharacterized by furnace cooling. Air cooling or quenching aresufficient to carry the alloy through the 10001600F range for ordinarythicknesses quickly enough to avoid precipitation of detrimental phasessuch as sigma phase.

DETAILED DESCRIPTION OF THE INVENTION The accompanying drawing is a plotof the 1.0% nitrogen section of the iron-chromium-manganesenitrogenquaternary phase diagram of an alloy that also contains 1% copper.

Copper does not have a significant effect on austenitic structure andaccordingly doesnt appear as a factor in equation (1) dry forth above,which defines the line l-A of the drawing. The area above line l-Agenerally represents compositions where a two-phase alloy of austeniteand ferrite exist. As mentioned above, this two-phase system isundesirable because it does not have the good mechanical or chemicalproperties of a single phase austenitic alloy. The area below line l-Ais a single phase austenitic alloy. I

Equation (2) defines the line 2-B. The area below the line 2-Brepresents compositions where nitrogen comes out of solution duringsolidification and creates porous ingots. The area above line 2-B iswhere nitrogen remains in solution during solidification and non-porousingots are formed.

The area A-C-B therefore represents the area in which the alloys of thisinvention fall for this particular cross section of the quaternary phasediagram. Lines LA and 2-B are the best lines to represent mildlyscattered data obtained from analysis of 26 heats which included asignificant number in the porous ingot zone, in the two-phaseaustenite-ferrite zone and in the onepha'se non-porous austenite zone.

To demonstrate the benefits of this invention, four alloys were preparedhaving compositions set forth in Table One.

TABLE ONE Alloy l is not an alloy in accordance with this inventionbecause its copper content is substantially below 1% and in fact is atthe residual level. All other alloys are within the scope of thisinvention containing nominally 1%, 2% and 3% copper.

All heats were melted in an air induction furnace and cast to ingot formwithout the occurrence of ingot porosity. Microscopic examination ofspecimens from each ingot revealed that all had austenitic structure.Alloys 3 and 4 showed some segregation of copper at interdendriticregions. When alloys 3 and 4 were hot rolled, they cracked in the mannerof hot shortness.

Ingots of alloys .1 and 2 were hot rolled, annealed at 1950F for aperiod of 60 minutes per inch of cross section thickness, were then coldrolled to a 50% thickness reduction and finally annealed again at 1950Ffor a period of 60 minutes per inch of thickness. The annealed stripswere never furnace cooled in order to bring them through the lO0-1600Ftemperature range slowly and as a result the finally rolled, annealedstrips were austenitic. The strips were cleaned by the usual techniques,a number of specimens taken from each and their properties measured. Themechanical properties of alloys 1 and 2 are set forth in Table Two.

TABLE TWO PROPERTIES ALLOY 1 ALLOY 2 Tensile strength as annealed 0.2%yield Strength (KS1) longitudinal 97.5-99.2 1083-1089 transverse982-1005 ll 1.4-1 1 1.6 Ultimate tensile strength (KS1) longitudinal146.1l46.4 l52.8-152.9 transverse 147.7-149.2 l56.0-157.3 Elongationlongitudinal 50-52 44-46 transverse 45-50 44.5-46 Hardness (Rockwell) 2831 Tensile strength after cold reduction of annealed strip 10% reduction0.2% yield strength (KS1) longitudinal 158.2-1 62.0 121 .9-1 26.9transverse 151.4151.8 119.6-1 19.9 Ultimate tensile strength KS1)longitudinal 186.6-187.9 163.6-163.7 transverse 184.7-l91.2 163.0163.1Elongation longitudinal 28-30 36-38 transverse 27.5-30 37.5-38 Hardness(Rockwell) 40 36.5 25% reduction 7 0.2% yield strength (KS1)longitudinal 201 .9204.4 191 .9-192.6 transverse 193.0-194.8 183.3-183.8Ultimate tensile strength (KSI) longitudinal 21 8.82l9.4 215.9- 217.0transverse 222.4-224.2 2l5.42l8.8 Elongation longitudinal 18- l 9.5 18transverse 13-155 16 Hardness (Rockwell) 46 45.5 50% reduction 0.2%yield strength (KSI) longitudinal 256.5-263.8 254.0-256.5 transverse246.7-250.3 224.6-226.1 Ultimate tensile strength KS1) longitudinal275.3283.8 276.6277.0 transverse 288.6-289.6 262.7264.6 Elongation vlongitudinal 7-7.5 3-4 transverse 8.0 7.5-8.0 Hardness (Rockwell) 49.849.5 Strain Hardening Exponent 0.36 0.36

From the data in Table Two it is evident that alloys of this inventionwith a copper content of 1% are capable of being worked. It is alsoevident that the presence of copper in the alloy has no significanteffect on the mechanical properties, of the worked alloy.

Specimens of alloy 1 and 2 were subjected to standard corrosion testingto measure their resistance to sulfuric acid. Resistance of an alloy tosulfuric acid is measured by exposing an alloy specimen in sulfuric acidand obtaining both the anodic polarization data and 1 the cathodicpolarization data and determining theirintersection point on the sameplot of voltage versus current. A correlation is known to exist betweenthe intersection points of these lines and the resistance of thespecimen to sulfuric acid corrosion. Intersection of these lines atlower current flows indicates better resistance to sulfuric acid.

Table Three contains data obtained in the foregoing test on specimens ofalloy 1 and alloy 2. The resistance to sulfuric acid corrosion wasmeasured at three different sulfuric acid concentrations, namely, 1.0normal, 5.0 normal and 10 normal.

TABLE THREE acid or to attack by chlorides than alloy 2. What is claimedis: 1. An alloy consisting essentiall of about 15-45% manganese, about-30% chromium, about l3% Anodic-cfllhodic Polarization 5 copper, aboutOBS-3% nitrogen, about 0-l% carbon, m N H250 'mersecuon about 0-2%silicon and the balance iron and residuals ALLOY 1 ALLOY 2 at(milliamp/cm") 5.0 .012

5.0 N H so, intersection wherein the composition is such that:

at (milliamp/cm 9.0 .012

10.0 N H.2804 intersection at (milliamp/cm 1.50

Table Three indicates that an alloy of this invention 0 r a l) with aminimum copper concentration, 1%, has signifi- %Cr M 88(%N Q1) (285cantly greater resistance to sulfuric acid than the prior %Cu) 2 0 (2)art alloys containing only residual amounts of copper. A hot workablealloy in accordance with claim 1 The two order of magnitude differencein intersection mpri ing o abOUt l -5 copper.

points represents a very significant difference in sus- An ll y in ac oa ce ith claim 1 comprising ceptibility to sulfuric acid corrosion. Theintersection 21-30 nganesepoint of alloy 1 was not measured in the 10normal sul- 4. An alloy in accordance with claim 1 comprising furic acidsolution because the current at which the in- 15-27% chromium.tersection occurred would be too large to be meaning- 5. An alloy inaccordance with claim 1 comprising ful. 105-1 .5% nitrogen.

When subjected to tests to measure susceptibility of 6. An alloy inaccordance with claim 1 comprising the alloys to chloride pitting, itwas found that both 00.15% carbon. alloy 1 and alloy 2 have highresistance to chloride pit- 7. An alloy in accordance with claim 1comprising ting and are about equivalent in this regard. 0-l% silicon.

Alloys 3 and 4 are more resistant to attack by sulfuric

1. AN ALLOY CONSISTING ESSENTIAL OF ABOUT 15-45% MANGANESE, ABOUT 10-30%CHROMIUM, ABOUT 1-3% COPPER, ABOUT 0.85-3% NITROGEN, ABOUT 0-1% CARBON,ABOUT 0-2% SILICON AND THE BALANCE IRON AND RESICUALS WHEREIN THECOMPOSITION IS SUCH THAT:
 2. A hot workable alloy in accordance withclaim 1 comprising from about 1-1.5% copper.
 3. An alloy in accordancewith claim 1 comprising 21-30% manganese.
 4. An alloy in accordance withclaim 1 comprising 15-27% chromium.
 5. An alloy in accordance with claim1 comprising 1.05-1.5% nitrogen.
 6. An alloy in accordance with claim 1comprising 0-0.15% carbon.
 7. An alloy in accordance with claim 1comprising 0-1% silicon.